The use of products formed from polytetrafluoroethylene (PTFE) tubes in medical applications is well known. Products such as implantable grafts, covered stents, catheter tubing and the like may be formed from tubes of PTFE. One technique for forming PTFE tubing is to use a wet flow paste extrusion process. The paste extrusion process yields a green tube which is then subjected to secondary operations such as heating, expansion and sintering to yield a porous expanded PTFE (ePTFE) tube having a porous node and fibril structure which optimizes its use in medical applications.
Often, the green tube produced by the paste extrusion process has a relatively thick wall and a small internal diameter. The secondary operations of expanding the tube and subsequently sintering the tube results in a certain degree of thinning of the wall of the tube. However, the expansion process currently being used limits the amount of thinning or circumferential orientation that can be achieved in the wall thickness of the tube. In certain applications, the resultant ePTFE tube may still exhibit a sufficiently large wall thickness making it difficult to use in certain medical applications. Additionally, extrusion process results in the extruded tube being highly oriented in a single (longitudinal) direction. Thus the tube would exhibit reduced strength in the transverse or circumferential direction. The secondary operations do little to improve the circumferential strength of the tube.
In instances where an extruded ePTFE tube is being used to form an implantable luminal prosthesis, the thickness and circumferential strength of the tube are of particular concern.
For example, a relatively thick tube may be more difficult to implant, especially where the prosthesis is designed for endoluminal delivery. Furthermore, in situations which require multiple layer prostheses, such as with a graft formed by a tube-over-tube construction, or where the ePTFE tube is being used to form a stent/graft composite structure having a stent employed in combination with one or more graft layers, the wall thickness of the ePTFE tube is of particular concern. With grafts formed of multiple layered tubes, the porosity of the graft may be affected by the thickness of the structure. In stent/graft composite structures, the thickness of the ePTFE tube may render the delivery and deployment of the composite structure difficult. Moreover, circumferential strength is required both during implantation as well as in use. The prosthesis must be capable of withstanding expansion upon deployment and must also withstand the internal pressure of blood flow in use.
It is, therefore, desirable to provide a method and apparatus for forming a PTFE green tube of reduced wall thickness that imparts a degree of circumferential orientation to a PTFE green tube.